The present invention generally relates to the production of gem-quality diamonds and more particularly to the production of these gem-quality diamonds from inferior-grade discolored or so-called “brown” diamonds.
Diamonds are conventionally divided into four main categories which are designated as Type Ia, Type Ib, Type IIa, and Type IIb. In reality, there is a smooth change in impurity concentration/arrangement between the four types so that intermediate varieties thereof also exist. Type I diamonds contain nitrogen as the major impurity. This category is divided into Type Ia diamonds where the nitrogen exists in an agglomerated state as either pairs called A Centers (Type IaA) or clusters of four nitrogen atoms called B centers (Type IaB) or mixtures thereof (Type IaA/B), and Type Ib where the nitrogen occurs as only isolated single nitrogen atoms called C Centers. Some diamonds also contain clusters of three nitrogen atoms called N3 Centers. Over 98% of the larger clear natural diamonds are Type Ia. Type Ib diamonds are rarer and amount to only 0.8% of natural stones. Type Ia diamonds also contain platelets, which are small flat inclusions a few atoms thick and about 300 atoms across, that may contain some nitrogen in an unspecified form. Type Ia diamonds also may contain voidites which are small equiaxed cavities that are either vacant or which contain nitrogen in an unknown form. Voidites tend to be seen principally in Type IaA/B or Type IaB diamonds.
Generally, it is believed that all nitrogen-containing diamonds started out as Type Ib with isolated nitrogen atoms (C Centers) that were incorporated during crystal growth. During a long period of time (perhaps up to 1 billion years), the diamonds were annealed within earth's mantel at temperatures between 1000° and 1300° C. and at high pressure. During this time, the nitrogen atoms migrated in the diamonds and principally formed two types of aggregates, namely pairs (A Centers) or clusters of four (B Centers).4C→2A→1B  (1)
It is believed that the clusters of four nitrogen atoms (B Centers) are formed when migrating nitrogen pairs (A Centers) collide with each other. Thus, the progression is believed to be Type Ib to Type IaA to Type IaA/B to Type IaB. A small amount of nitrogen may also agglomerate as N3 centers which are a planar array of three nitrogen atoms surrounding a common vacancy. It is believed that such centers are formed when an isolated nitrogen (C Center) combines with a nitrogen pair (A Center) during the agglomeration process. N3 centers apparently are less stable than A and B centers as their concentration in Type Ia diamonds is relatively small. Platelets form as soon as the annealing has progressed to the Type IaA stage. Voidite formation, as well as some platelet disintegration, occurs as B clusters form and becomes pronounced in the Type IaB stage of annealing.
Type II diamonds make up approximately 0.7% of all diamonds and contain no nitrogen. Type II diamonds are further divided into Type IIa and Type IIb. Type IIa diamonds have no impurities. Type IIb diamonds contain boron in the parts per million range and are extremely rare, comprising less than 0.001% of all diamonds.
The color of natural diamonds can range from clear and colorless to yellow, orange, red, blue, brown, and even green. For natural diamonds, a brownish tinge is the most common color and may occur in as many as 98% of mined natural diamonds. Type Ia diamonds containing nitrogen can be colorless if all of the nitrogen is tied up in A or B centers. However, if isolated nitrogen atoms (C Centers) or N3 centers are present, the diamonds will have a yellow tinge whose hue depends on the concentration of these forms of nitrogen atoms. Typically, the N3 centers produce the washed-out yellow that is referred to as “Cape Yellow,” while isolated nitrogen atoms (C Centers) produce the richer more vibrant “Canary Yellow” if their concentration is high enough. A small amount of yellow in an otherwise colorless diamond can significantly decrease its market price. However, a rich deep yellow color can produce a “fancy” yellow that has a very high value in the marketplace.
Changing the concentration of N3 centers not only will change the yellow color of a diamond, but can increase the actual brilliance or amount of light thrown back by the diamond. The electrons around an N3 center absorb light in the invisible ultraviolet part of the natural light spectrum, as well as blue light in the visible spectrum. In normal daylight, about 1/10 of the energy of the light is in the form of invisible ultraviolet radiation. If the N3 concentration is relatively high, i.e., 100 ppm, then visible blue light is strongly absorbed and the diamond will have a definite yellow color, which will lower its value. However, if the concentration of N3 centers is reduced by some treatment so that the yellow coloring disappears, the remaining N3 centers can affect the brilliance of a diamond by a two-stage process. First, an invisible ultraviolet photon is absorbed by an N3 center. The energy is temporarily stored in the N3 center. Some of this energy leaks away in the form of phonons or lattice vibrations. After a storage time pre-determined by the half-life of the center, the N3 center will re-emit the remaining energy as visible light. Since some energy has been lost, the re-emitted light is no longer in the high-energy invisible ultraviolet part of the spectrum. Instead, the re-emitted light now is in the visible spectrum (the technical term for this is “ultraviolet downshifting”). Because we do not see ultraviolet light, we do not notice that it is being absorbed (an animal, like a bee that can see ultraviolet light, would see the brilliance of the diamond decreased by the absorption of ultraviolet light by N3 centers). All we see is the increased emission in the visible spectrum and, thus, the diamond now appears extraordinarily bright. Consequently, a controlled reduction of N3 centers in a Type Ia diamond by any treatment will increase the value of a diamond containing them in two ways. First, elimination of some N3 centers reduces or eliminates the yellow tinge in the diamond. Second, the remaining N3 centers will increase the brilliance of the diamond relative to a perfect Type IIa diamond.
Most Type Ia diamonds as mined are of a brownish color. A brown color can result from the mixture of many other fundamental colors. One way is to mix some yellow coloring from isolated nitrogen atoms (C Centers) or N3 centers with some black color, perhaps from submicroscopic inclusions of graphite. The mixture of yellow and black will produce a brown color. Another way to make a brown diamond is to mix a color center that produces a green diamond with a color center that produces a red diamond. The combination of red and green again will produce a brown color. In fact, there are an infinite number of color combinations that will produce a brown color. Thus, it is not possible to determine the color centers causing the color of a diamond by its color. However, the reverse process is unique; that is, if one knows the type and concentration of color centers in a diamond, one can predict the resulting color.
Type II diamonds vary from colorless to a deep blue color. Type II diamonds are most valuable when they are colorless or a pure-blue color. Excessive mechanical deformation and plastic flow are believed to cause many Type II diamonds to have a reddish brown or pink tinge which lowers their value considerably. Many natural Type IIa diamonds have this color tinge and their value could be greatly enhanced as jewelry if they could be made colorless. Some Type IIa diamonds have a steel gray haze in them that also greatly decreases their value. Generally, in the diamond trade, a discolored diamond whether it is brownish, reddish brown or steel gray is labeled as a “brown” diamond. Previous attempts to treat Type IIa diamonds to increase their value have failed. G. Lenzen, Diamonds and Diamond Grading, p. 207, Buttersworth, London (1983). Both neutron and electron irradiation followed by annealing caused Type IIa diamonds to turn brown, thereby greatly lowering the value of the diamonds.
Type IIb diamonds have a blue color that is imparted by the boron impurity acting as an electron acceptor. Because of the rarity of Type IIb diamonds and their attractive blue color, they have the highest value per carat as jewelry items.
In general, the pricing of diamonds is a sensitive function of their color. Fancy-color diamonds, such as the canary yellows, blue, red, pink and green diamonds, are rare and have the highest prices of any diamonds. Because of their rarity, the market for them is not well organized and they are usually sold via a Sotheby's or Christie's type of auction. Brown diamonds are an exception to the fancy color diamond market. Brown diamonds are very common and in the past have been culled and used as industrial diamonds and are correspondingly inexpensive. After fancy color diamonds, colorless diamonds command the highest prices. The degree of colorlessness has a strong nonlinear effect on the price of the diamond. Even the faintest tinge of yellow can considerably reduce the price of colorless diamonds.
In view of the relative prices of fancy colors, colorless, and brown diamonds, there is a strong financial incentive to change brown diamonds to either colorless diamonds or to fancy color diamonds. Irradiation has been used frequently to change the color of such diamonds from unattractive off-colors to attractive blue, green, orange, black, and yellow colors. Electrons, neutrons, gamma rays, and alpha particles have been used to produce irradiation-produced color centers in diamond. Neutron, gamma, and electron irradiation are preferred because they produce a more uniform coloration of the diamond because of their good penetrating power. There is some danger in using neutrons since radioactive species can be produced in inclusions in diamonds by neutron activation. In addition, the most common electron or alpha-irradiation treatments only develop a superficial color confined to the outer portions of the diamond.
Essentially, all of the different types of radiation produce vacancies in diamond which are seen as the GR1 band in the visible spectrum. Absorption by the GR1 band produces a green, blue-green, dark green, or even a black color in the diamond by absorption at 740.9 nm and from 412–430 nm. Vacancy-color centers can be modified by high-temperature annealing to produce colors ranging from blue to pink to red to green. Annealing can be done at temperatures as low as 600° C., because the large number of vacancies introduced by irradiation temporarily increases the mobility of nitrogen and other impurities in the diamond. Eventually, the vacancies diffuse to and are absorbed by vacancy sinks, such as free surfaces, dislocations, and inclusion interfaces in the diamond. Naturally, as the vacancies disappear, their direct effect on the color of the diamond also lessens. Thus, the color gradually goes through a sequence of one from blue to green to brown to yellow and back to the original color of the diamond. The annealing can be stopped at any point of the annealing sequence to produce the color desired. Multiple irradiation steps and annealing steps can be done to further manipulate the color.
In recent years, people have annealed diamonds at progressively higher temperatures to try to eliminate telltale signs of irradiation in the diamond because “treated” diamonds are valued at a discount to natural diamonds. The GR1 line from the vacancy begins to disappear above 400° C. as the vacancies anneal out of the crystal. Other irradiation lines, however, persist to higher temperatures. Much of the information concerning diamond irradiation and annealing treatments is kept as trade secrets by the organizations carrying out such treatments.
An example of a possibly irradiated and heat-treated greenish-yellow diamond was reported in a recent issue of Gems & Gemology, XXXIII, pp. 136–137, (Summer, 1997). Several one-carat round brilliant stones were given to the GIA laboratory for testing. From their spectroscopic studies, GIA concluded that these diamonds had been treated. In addition, they inferred that the diamonds had been irradiated and subsequently heated to above 1450° C. Although the normal irradiation signatures, such as the GR1 line at 741 nm and the HIb and HIc lines arising from a combination of irradiation and heat treatment, were absent in these two stones, the stones did have an absorption peak in the near-infrared range at 985 nm. Although the detection of treated stones is more of an art than a science, it is commonly believed that diamonds that show no absorption peaks at 595 nm, 1936 nm, and 2024 nm have “almost certainly not been treated.” J. Wilks, et al., Properties and Applications of Diamonds, p. 91, Buttersworth, London (1991).
Type Ia diamonds in which N3 centers give a slight yellow tinge to the crystal have been the most commonly selected stones for irradiation and annealing treatments. Electron or neutron irradiation of these stones and a subsequent heat treatment generates H3 (Nitrogen-Vacancy-Nitrogen) and H4 (Nitrogen-Nitrogen-Vacancy-Nitrogen-Nitrogen) centers which give a pleasing amber-gold color to the stones when exposed to light of the visible spectrum. It has been found that diamonds that do not luminesce produce more attractive colors than diamonds that luminesce. A. T. Collins, J. Gemology, XVIII, pp. 37–75 (1982). Apparently, existing color centers in the stone add to the color produced by the irradiation and heat treatment, and the resulting color is less desirable.
Changing the concentration of N3 centers not only will change the yellow color of a diamond, but can increase the actual brilliance or amount of light thrown back by the diamond. The electrons around an N3 center absorb light in the ultraviolet part of the natural light spectrum as well as blue light in the visible spectrum. In normal daylight, about ⅕ of the energy of the light is in the form of ultraviolet radiation. If the N3 concentration is relatively high, i.e., 100 ppm, then visible blue light is strongly absorbed and the diamond will have a definite yellow color, which will lower its value. However, if the concentration of N3 centers is reduced by some treatment so that the yellow coloring disappears, the remaining N3 centers can affect the brilliance of a diamond by a two-stage process. First, an ultraviolet photon is absorbed by an N3 center. The energy is temporarily stored in the N3 center. Some of this energy leaks away in the form of phonons or lattice vibrations. After a storage time pre-determined by the half-life of the center, the N3 center will re-emit the remaining energy as light. Since some energy has been lost, the re-emitted light is no longer in the high-energy ultraviolet part of the spectrum. Instead, the re-emitted light now is in the visible spectrum (the technical term for this is “ultraviolet downshifting”). Because we do not see ultraviolet light, we do not notice that it is being absorbed (an animal, like a bee, that can see ultraviolet light would see the brilliance of the diamond decreased by the absorption of ultraviolet light by N3 centers). All we see is the increased emission in the visible spectrum and, thus, the diamond now appears extraordinarily bright. Consequently, a controlled reduction of N3 centers in a Type Ia diamond by any treatment will increase the value of a diamond containing them in two ways. First, elimination of some N3 centers reduces or eliminates the yellow tinge in the diamond. Second, the remaining N3 centers will increase the brilliance of the diamond relative to a perfect Type IIa diamond.
Another approach that has been tried to alter the color of a natural type Ia diamond is to go to very high temperatures and pressures where nitrogen atoms are more mobile. For each 100° Centigrade increase in temperature, the mobility of nitrogen in diamond increases almost an order of magnitude. See Evans, et al., Proc Roy Soc Lond, a 344, 111–130 (1975) and Bonzel, et al., Proc Roy Soc Lond, A 361, 109–127 (1978), discussing annealed Type Ia diamonds containing nitrogen at temperatures above 1960° C. under stabilizing pressures as high as 85 kilobars (kbars). The application of pressure is necessary to keep the diamond in the diamond-stable part of the Pressure-Temperature diagram of carbon (carbon-phase diagram). F. P. Bundy, Physica, A156, 169–178 (1989). Otherwise, it is reported that exposure of diamond to such high temperatures would result in the rapid graphitization of the diamond. The diamond-stable phase versus the graphite-stable phase is generally accepted as being defined by the Simon-Berman line. The Simon-Berman line separates the diamond and graphite-stable regions on the phase/temperature (PT) plot. This line may also be referred to as the diamond-graphite equilibrium line. C. S. Kennedy and G. C. Kennedy in J. Geophysics Res, Vol. 81, pp. 2467–2469 (1976) define the Simon-Berman line by the equation:P(kilobars)=19.4+0.025 T (degrees Centigrade).
Most of the diamonds that have been treated by Evans et al. and Bonzel et al. were of the type IaA/B, i.e., they had a mixture of nitrogen clusters comprised of either nitrogen pairs (A Centers) or quadruples (B Centers) since diamonds with either pure IaA or IaB characteristics are very rare. All of the diamonds contained platelets. In the diamonds with predominantly A-Center clusters, the diamonds turned a yellow color as some of the clusters broke up and formed C Centers, isolating nitrogen atoms (Type Ib). They were less successful in treating diamonds with predominantly B Centers which apparently are more stable than A Centers. The most attractive and deepest yellow colors were obtained with Type Ia diamonds at temperatures between 2250° and 2300° C. and 48 kilobars of pressure (Evans et al., supra.).
Although Evans and co-workers achieved a successful color change, both the Type Ia and IIa diamonds crumbled into small pieces. In other words, the operation was successful but the patient died. Nothing of economic value was created and any original value of the diamonds was destroyed by the treatment. As a result of their work, high-pressure and high-temperature treatments of diamond to change their color were abandoned by the diamond research community in favor of irradiation and low-temperature annealing.